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          <h1 class="post-title" itemprop="name headline">什么是IAP编程</h1>
        

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        <p>本文以arm-cortex-M4架构为例来探讨IAP编程，对于细节的flash的分块编译链接部分不子在此作详细阐述,后续再单独分析。</p>
<a id="more"></a>

<hr>
<h2 id="什么是IAP"><a href="#什么是IAP" class="headerlink" title="什么是IAP?"></a>什么是IAP?</h2><p><strong>IAP</strong>：应用程序自编程，即实现应用程序对自身的升级。通常对自身升级的程序叫做bootloader(以下简称boot),被升级的程序叫做app程序(以下简称app)。</p>
<p>MCU在出厂时,厂家已经自带了boot,所以我们能通过串口/USB等外部接口实现程序升级。但是这部分的协议，厂商是不开放的，所以为了产品的持续迭,我们就需要去实现自己的boot,以便在需要的时候,我们可以通过自己的方式实现程序的升级。</p>
<p>下图是STM32F4XX系列MCU的flash构成图,系统存储器区域即厂家Boot程序区，我们使用厂家下载器烧录程序时,程序就是从这里运行的。iap编程就是我们要将主存储器区分为boot区和app区，升级时程序在boot区运行，实现功能逻辑时，程序在app区运行。</p>
<p><img src="/blog/2020/04/22/what_is_iap/STM32F4xxflash.png" alt="asset"></p>
<p>图1：STM32F4XX(STM32F40x和STM32F41X) Flash模块构成图</p>
<h2 id="怎么实现bootloader"><a href="#怎么实现bootloader" class="headerlink" title="怎么实现bootloader?"></a>怎么实现bootloader?</h2><p>为了去实现bootloader,接下来从boot程序和app程序分别来介绍。下图是一种flash分区方式,boot程序占用32kb，剩下的留给app程序。程序刚启动时，从0x800000地址开始执行，若没有升级的请求，则跳转到0x8008000执行app程序。</p>
<p><img src="/blog/2020/04/22/what_is_iap/flash.png" alt="asset"><br>图2：一种flash分区方法</p>
<h3 id="bootloader程序"><a href="#bootloader程序" class="headerlink" title="bootloader程序"></a>bootloader程序</h3><p>boot程序主要实现对app程序区flash的读写,以及绝对地址的跳转。</p>
<h4 id="flash的分区"><a href="#flash的分区" class="headerlink" title="flash的分区"></a>flash的分区</h4><p>这部分的功能是编译链来实现的,在不同编译环境下设置方式不同。这里以arm-cc的keil为例，后续写到makefile之后再回头完善。<br>flash大小设置为32kb</p>
<p><img src="/blog/2020/04/22/what_is_iap/bootflash.png" alt="asset"></p>
<p>图3：bootflash设置</p>
<p>链接使用target中设置的参数，否则它会使用默认脚本进行链接</p>
<p><img src="/blog/2020/04/22/what_is_iap/link.png" alt="asset"></p>
<p>图4：链接设置</p>
<h4 id="flash的擦除和读写"><a href="#flash的擦除和读写" class="headerlink" title="flash的擦除和读写"></a>flash的擦除和读写</h4><p>这个主要以原厂提供的方式来进行操作，需要注意的是flash的特性写入只能由1写成0,即如果要写入成功,必须先进行擦除，每次擦除的最小单位为一块。</p>
<h4 id="绝对地址跳转"><a href="#绝对地址跳转" class="headerlink" title="绝对地址跳转"></a>绝对地址跳转</h4><p>实现程序从boot程序跳转到应用程序，在跳转之前可以做一些栈顶地址检测或者应用程序完整性校验的判断，通过后才能跳转，跳转地址为app程序的复位向量0x8008004(arm-cortex系列为基地址偏移4)。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">void Jump( uint32_t TargetAddr )</span><br><span class="line">&#123;</span><br><span class="line">        unsigned long* ucaddr;</span><br><span class="line">        ucaddr &#x3D; ( unsigned long* )( TargetAddr );</span><br><span class="line">        ( *( ( void( * )( void ) )( *ucaddr ) ) )();</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

<h4 id="常用在线升级的方式"><a href="#常用在线升级的方式" class="headerlink" title="常用在线升级的方式"></a>常用在线升级的方式</h4><p>通过串口、usb、以太网、wifi、蓝牙、SD卡等任何可以与MCU通讯的方式都可以实现在线升级，以下提供几种通讯过程中的机制,加强在线升级的可靠性。</p>
<h5 id="断点续传"><a href="#断点续传" class="headerlink" title="断点续传"></a>断点续传</h5><p>断点续传在通讯方式升级中很有必要性，特别是网络升级，一个小延时或断网都可导致设别变成砖。</p>
<p>升级之前,先判断应用程序的完整性，若非完整，则通过应用程序的长度计算CRC与上位机进行比对,比对通过,则可从断点处开始继续烧录，否则就需要重新烧录。</p>
<h5 id="程序备份"><a href="#程序备份" class="headerlink" title="程序备份"></a>程序备份</h5><p>1、通过外部flash或sd卡形式，对程序备份，一旦烧录失败，还可以恢复为之前的程序</p>
<p>2、对flash重新划分,分别为boot,app1,app2。只有app1校验通过,才执行app1,否则执行app2，app2为app1的备份区。</p>
<h5 id="程序加密"><a href="#程序加密" class="headerlink" title="程序加密"></a>程序加密</h5><p>1、文件型。通过对整体文件进行加密，如使用3des将bin文件加密为密文，再修改程序后缀为任意形式。boot程序读取该类型文件并进行解密之后再烧录。</p>
<p>2、通讯型。通讯内容使用对称加密，如3des，boot程序解密后再进行烧录。</p>
<h3 id="app程序"><a href="#app程序" class="headerlink" title="app程序"></a>app程序</h3><h4 id="flash的分区-1"><a href="#flash的分区-1" class="headerlink" title="flash的分区"></a>flash的分区</h4><p><img src="/blog/2020/04/22/what_is_iap/appflash.png" alt="asset"></p>
<p>图5：app程序flash设置</p>
<h4 id="中断向量重定位"><a href="#中断向量重定位" class="headerlink" title="中断向量重定位"></a>中断向量重定位</h4><p>中断向量的地址是程序再初始化之前就要确定的，因此程序跳转到应用程序后，需要对中断向量的地址进行重定位，否则产生中断时，程序就跳到了boot程序，造成程序执行错误。arm-cortex系列中断向量的重定位比较简单，只需要对基地址进行重新定义就好了，某些MCU比如瑞萨的RL系列，需要手动对每一个中断向量作重定位，就比较麻烦了。</p>
<hr>
<p>STM32F4中断向量重定位到0x8008000,就只需要将VECT_TAB_OFFSET定义为0x8000即可。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">SCB-&gt;VTOR &#x3D; FLASH_BASE | VECT_TAB_OFFSET;</span><br></pre></td></tr></table></figure>


<hr>
<h2 id="参考文献"><a href="#参考文献" class="headerlink" title="参考文献"></a>参考文献</h2><p>1、《cortex-M4技术参考手册》<br>2、《cortex-M4权威指南》<br>3、《STM32F4xx中文参考手册》</p>

      
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